1. Field of the Invention
The present invention relates generally to amplifiers, and, more particularly, relates to a method and apparatus for correcting distortion in amplifiers.
2. Description of the Related Art
Switching amplifiers are used to provide amplification of audio signals and other signals where efficient amplification is required while maintaining a relatively high efficiency is required. Such amplifiers are often referred to as Class-D amplifiers. Class-D amplifiers process analog signals using pulse width modulation (PWM), pulse duration modulation (PDM), pulse amplitude modulation (PAM), or the like. For example, in PWM Class-D amplifiers used to amplify audio signals, high-frequency rectangular waves of constant amplitude, but varying duty cycles, are output from an integrated circuit or from a circuit comprising multiple components. The rectangular waves of varying duty cycles contain the audio information. The output signal is low-pass filtered to isolate the audio information from the high-frequency switching signal to reproduce the original audio signal at an amplified magnitude. Because of their compact size, their relatively low power dissipation, and their relatively high efficiency, Class-D amplifiers and other switching amplifiers are finding extensive use in consumer audio electronics.
Although switching amplifiers are finding increasing use in consumer electronics, such amplifiers have historically been avoided for very high-end audio systems where very low distortion and very low noise are required. Because of the characteristics of switching amplifiers, switching amplifiers tend to have more distortion than conventional linear amplifiers. Historically, designers have had difficulty in implementing efficient error correction, or feedback, around the output stage to substantially reduce or eliminate the distortion. Thus, a need continues to exist for an amplifier having the low cost and high efficiency characteristics of a switching amplifier, but with the low distortion of a linear amplifier.
The present invention is responsive to the foregoing problem by providing an apparatus and a method for correcting the distortion in the output of a switching amplifier. In particular, the present invention uses a supplemental amplifier (e.g., a linear amplifier) coupled in series with a primary amplifier (e.g., a switching amplifier) to provide correction signals that not only attenuate in-band distortion, but also attenuate RF components that are not filtered out by the low-pass filter.
One aspect of the present invention is an apparatus that amplifies an input signal and that generates a low-distortion output signal. In the apparatus, a first amplifier (i.e., a primary amplifier) has a first input that receives an input signal to be amplified and has a first output that generates a first output signal that is an amplified reproduction of the input signal. The first output signal has distortion. A second amplifier (i.e., a supplemental or secondary amplifier) has a second input that receives an error signal. The second amplifier has a second output that generates a second output signal that is an amplified reproduction of the error signal. The first output of the first amplifier and the second output of the second amplifier are connected in series to generate a system output signal that is the sum of the first output signal and the second output signal. A correction circuit compares a signal responsive to the input signal to a signal responsive to at least the first output signal of the first amplifier and generates the error signal provided to the second amplifier. In certain configurations, the signal responsive to at least the first output signal is responsive to the sum of the first output signal and the second output signal. In alternative configurations, the signal responsive to at least the first output signal is responsive to only the first output signal.
In particular embodiments in accordance with this aspect of the invention, the first amplifier is a switching amplifier (e.g., a full-bridge switching amplifier or a half-bridge switching amplifier) and the second amplifier is a linear amplifier. Other combinations of amplifiers can also be advantageously incorporated into the present invention. In preferred embodiments, the system output signal is provided to a load, such as a transducer (e.g., a speaker in an audio system). In particular embodiments, the load is connected in series with the first amplifier and the second amplifier. In alternative embodiments, the load is connected in a bridge configuration between the first amplifier and the second amplifier.
Another aspect of the present invention is a method of generating a low-distortion output signal in response to an input signal. The method receives an input signal to be amplified. The method generates a first output signal that is an amplified reproduction of the input signal. The first output signal includes distortion. The method generates a second output signal that is an amplified reproduction of an error signal. The second output signal is generated in series with the first output signal, and the first output signal and the second output signal are added to generate a system output signal. The error signal is generated by comparing a signal responsive to at least the first output signal with a signal responsive to the input signal. In one embodiment of the method using feedback error correction, the signal responsive to at least the first output signal is responsive to the sum of the first output signal and the second output signal. In an alternative embodiment of the method using feedforward error correction, the signal responsive to at least the first output signal is responsive to only the first output signal.
Another aspect of the present invention is an apparatus that amplifies an input signal and that generates a low-distortion output signal. The apparatus comprises a first amplifier that receives an input signal to be amplified and that generates an first output voltage responsive to the input signal. A second amplifier is connected in series with the first amplifier. The second amplifier receives an error signal and generates a second output voltage responsive to the error signal. At least one output terminal provides a system output voltage equal to the sum of the first output voltage and the second output voltage.
Another aspect of the present invention is a method of generating a low-distortion output signal in response to an input signal. The method comprises receiving an input signal to be amplified. The method generates a first output voltage that is an amplified reproduction of the input signal with distortion. The method also generates a second output voltage that is an amplified reproduction of an error signal representing the distortion in the first output voltage. The second output voltage is generated in series with the first output voltage. The first output voltage and the second output voltage are added to generate a system output signal in which the distortion in the first output voltage is reduced by the second output voltage.
Another aspect of the present invention is an apparatus that amplifies an input signal and that generates an output signal. The apparatus comprises a first amplifier that receives an input signal to be amplified and that generates an first output voltage responsive to the input signal. The apparatus further comprises a second amplifier connected in series with the first amplifier. The second amplifier receives a signal responsive to the input signal and generates a second output voltage responsive to the input signal. At least one output terminal provides a system output voltage equal to the sum of the first output voltage and the second output voltage.